.

Reference is again made to the diagram, Fig. 12, which shows that, in
most cases of coffer-dams in combined aqueous and earth pressure, there
may be nearly equal, and in some cases even greater, loading toward the
bottom.

The writer also specifically states that in air the difference between
aqueous and earth pressure is plainly noted by the fact that bracing is
needed so frequently to hold back the earth while the air is keeping out
the water.

The lack of specific data is especially noticeable in the account of the
rise of the 6-ft. conduit at Toronto. It would be of great interest to
know with certainly the weight of the pipe per foot, and whether it was
properly bedded and properly back-filled. In all probability the
back-filling over certain areas was not properly done, and as the pipe
was exposed to an upward pressure of nearly 1600 lb. per ft., with
probably only 500 or 600 lb. of weight to counterbalance it, it can
readily be seen that it did not conform with the writer's general
suggestion, that structures not compactly, or only partially, buried,
should have a large factor of safety against the upward pressure.
Opposed to Mr. Thomson's experience in this instance is the fact that
oftentimes the tunnels under the East River approached very close to the
surface, with the material above them so soupy (owing to the escape of
compressed air) that their upper surfaces were temporarily in water, yet
there was no instance in which they rose, although some of them were
under excessive buoyant pressure.

It is also of interest to note, from the papers descriptive of the North
River Tunnel, that, with shield doors closed, the shield tended to rise,
while by opening the doors to take in muck the shield could be brought
down or kept down. The writer concurs with those who believe that the
rising of the shield with closed doors was due to the slightly greater
density of the material below, and was not in any way due to buoyancy.

Concerning the collapse of the bracing in the tunnel built under a
side-hill, the writer believes it was due to the fact that it was under
a sliding side-hill, and that, if it had been possible to have
back-filled over and above this tunnel to a very large extent, this
back-fill would have resulted in checking the sliding of material
against the tunnel, and the work would thereafter have been done with
safety. This is corroborated by Mr. Thomson's statement that the tunnel
was subsequentl